MachO: Fix out-of-bounds memory access in getString16

Summary:
This fixes the following tests when gcc is compiled with gcc8:

lld :: mach-o/do-not-emit-unwind-fde-arm64.yaml
lld :: mach-o/eh-frame-relocs-arm64.yaml

llvm.org/PR38096

Reviewers: lhames, kledzik, javed.absar

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D51547

llvm-svn: 341670